1. Field of the Invention
This invention relates to a method and apparatus for operating an electrolysis plant, specifically an aluminum electrolysis plant, in which the electrode spacing in each furnace is controlled as a function of furnace resistance, the latter being calculated by means of a computer device from the furnace current and voltage.
2. Description of the Prior Art
The specific energy consumption of an electrolysis plant is an indication of its economic efficiency, and is determined by the relationship E.sub.sp = U/k (Y.sub.i), wherein U represents the furnace voltage, k is a constant (k=0.3354 for an aluminum electrolysis furnace), and Y.sub.i represents the furnace current yield. The furnace current yield Y.sub.i is equal to the amount of metal produced, divided by the product of the furnace current and the constant k. The furnace resistance R, however, instead of furnace voltage U, may be utilized to control the furnace electrode spacing. Since the furnace voltage is equal to the sum of the polarization voltage U.sub.o and the voltage drop I.R, U.sub.o may be approximated by a constant. The furnace resistance is then given by the equation R = (U-U.sub.o)/I.
The control of electrode spacing in such furnaces as a function of furnace resistance is generally known in the art. In such prior art control methods, furnace current and the electrolysis bath voltage are periodically sampled by a control system which is common to several furnaces or, alternatively, by a computer, and mean furnace resistance values are calculated therefrom. If such mean values exceed predetermined limits, the electrode spacing is adjusted accordingly by either raising or lowering the furnace anodes. Such a method is described in detail in German Offenlegungsschrift No. 1,925,201. Such methods, however, are disadvantageous in that even during normal furnace operation, the instantaneous furnace resistance may fluctuate considerably from the mean values calculated, not only due to statistical resistance variations, but also due to defective furnace operation and operating conditions, such as, for example, those which result from detached anode carbon, or from the so called "anode effect".
The above described prior art operating methods all compare the calculated mean furnace resistance values with the limits selected therefor to determine whether or not the furnace resistance is found to be within normal operating limits. The actual furnace resistance is then compared with a desired value therefor, and is adjusted accordingly by moving the furnace anodes as described above.
Another method of determining furnace resistance known in the art is to calculate the number of amplitude variations of an ac voltage which is superimposed upon the furnace voltage. Generally speaking, these variations are determined within a frequency range of from 1-20 Hz. Variations which occur within these upper and lower limits, but which exceed a specified signal level therebetween, are converted to a current which is proportional thereto. For further details regarding this method, reference may be had to German Auslegeschrift No. 1,242,884.